Immunization of cotton rats with vaccinia RSV F and G recombinants induced resistance to RSV challenge without potentiating disease even in animals previously administered varying amounts of RSV antiserum. Passive transfer of respiratory syncytial virus (RSV) antiserum suppressed the immune response to the RSV fusion (F) and large (G) glycoproteins expressed by recombinant vaccinia viruses administered intradermally. The suppressive effect of passively administered RSV immune serum could be largely overcome by administering recombinant vaccinia virus intranasally or by inoculating a large quantity of purified F and G glycoproteins intramuscularly. Immunization of cotton rats with highly purified fusion (F) and large (G) glycoproteins of respiratory syncytial virus induced resistance to RSV challenge without potentiating disease. This suggests that purified glycoproteins can be used safely in a vaccine against RSV. Although, the F and G glycoprotein vaccine induced a high titer of F and G specific antibodies these antibodies had low neutralizing activity. Systemic or topical administration of RSV hyperimmune antiserum to cotton rats hastened recovery from infection. It was found that ADCC and complement were not required for the full therapeutic effect of the passively transferred RSV antibody which suggest that virus neutralization is the major mechanism by which the antiviral effect is achieved.